The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
There is a considerable demand for electric powered transportation. Due to the current limitations of rechargeable batteries (energy density—220 Wh/Kg-300 Wh/Kg, depth of discharge, charge/discharge rates, and cycle-life issues), the general sequence of battery-powered vehicles' market entry (discounting slow short-range vehicles such as golf carts) is as follows:                Automobiles—these are the easiest vehicles to adopt electric power. Automobiles can accept heavy batteries, there is a relatively low battery drain rate, and operation can be safely stopped with depletion of the battery.        Powered sailplanes—Here a powerplant is used for launching an otherwise safe glider.        Fixed-wing training aircraft—useful for flights of short duration, operated from established airports with professional instructors, maintenance, and management.        Privately owned fixed-wing—next easiest due rolling take-off and landing with a high wingborne lift to drag ratio.        Electric VTOL (eVTOL)—more challenging due the high power required for hover, especially if high-speed efficient cruise is also required        
eVTOL has become even more challenging as the market shift from “specialized transport” aircraft making shorter trips (25-60 miles) between well-equipped terminals, to “urban mobility” aircraft making longer trips with at least one poorly equipped landing spot. The need is exemplified by information published by Uber™, and reproduced herein as prior art FIGS. 1A and 1B. FIG. 1A is a conceptual image of an upcoming urban transportation market for Uber®'s proposed hybrid-electric vertical takeoff and landing (eVTOL) aircraft. FIG. 1B is a projected schedule of development and operations for such aircraft.
Safety and efficiency are perhaps the two most critical factors for developing eVTOL aircraft to satisfy this market. To achieve high safety, much of the prior art is focusing on aircraft that use six, eight, or even more, independently operated rotors. If any single rotor fails in such aircraft, the other rotors are likely to be capable of making a safe landing. Even quad rotor aircraft are not considered to be particularly fault-tolerant, because failure of a single rotor can crash the aircraft.
Several proposed and prototype aircraft are being designed using this many-rotor strategy. For example, FIGS. 2A and 2B are artist's renditions of a prior art 16-rotor Volocopter™, FIG. 3 is a photograph of a prior art 8-rotor Ehang™, and FIG. 4 is a photograph of a prior art 8-rotor CityAirbus™. All these designs are, however, problematic because the rotors do not tilt from vertical lift to forward propulsion positions, and there are no wings. That combination is extremely inefficient in forward flight, which limits the aircraft to relatively short ranges.
Some eVTOL aircraft are being developed that continue to use the many-rotor strategy, but add a wing to improve forward flight efficiency. For example, FIG. 5 is an artist's rendition of a prior art 36-rotor Lilium™ eVTOL, in which the rotors tilt about the forward and aft wings. The manufacturer claims a 300 km range, and 300 km/hr speed. This aircraft is, however, still problematic because the high disc loading results in low power loading (high installed power per weight), which reduces efficiency and range, and produces high noise levels.
Instead of having the rotors tilt about the wings, it is possible to have the rotors disposed in fixed position with respect to the wings, and tilt the wings. An example of that strategy is shown in FIG. 6, which is an artist's rendition of an 8-rotor Airbus™ A3 Vahana. This aircraft is problematic because it trades off higher efficiency in forward flight for very high power requirements during transition from vertical lift to forward flight. In such transition, the wings act as huge airbrakes.
It is also possible to have the rotors tilt about one or more fixed wings. Although a photograph is not available, FIG. 7 is an image of a Computational Fluid Dynamics (CFD) flow solution for the Joby™ 6-rotor eVTOL concept. This aircraft resolves some of the problems cited above, but the use of many-rotor strategy means the rotors are relatively small. This necessarily means high disc loading, which results in low power loading (high installed power per weight) and high noise level.
The only other solution that the prior art seems to have contemplated is to separate the vertical lift rotors from the forward propulsion rotors/propellers. The idea is that use of different lift and cruise propulsion systems allows each system to be is optimized for its particular function. FIG. 8 is an artist's rendition of the Aurora™ eVTOL concept, which uses eight lifting rotors and an aft facing propeller. This design is problematic because the duplicate propulsion systems require heavier and more expensive hardware, have marginal climb rate in wing borne cruise due to sizing the cruise powerplant for level cruise, and potentially have a smaller wingborne stall speed margin—gust entry and recovery, due to design optimization for higher cruise lift coefficient (smaller wing). Example: if flying at 130 mph, a vertical gust of 20 Ft/sec will increase the angle of attack by 6 degrees, may stall a small wing at its efficient lift coefficient of 0.9, but not stall a bigger wing at CL=0.5.
FIGS. 9A and 9B show artists' conceptions of a similar design, the Terrafugia™ eVTOL. This design has the drawbacks mentioned above with respect to duplicate propulsion systems, and in addition, the twin tilt rotor configuration does not provide a method for pitch control in rotor borne flight.
Motor installations in the prior art are also directed towards small rotors having small torque requirements. For example, FIG. 10 shows the motor installation of the Airbus™ A3 Vahana™. The motor is arranged in a direct-drive configuration where motor and propeller spin at the same rotational speed. This simple propulsion system solution is problematic for large rotors with large torque requirements.
Because of the physics involved, it is relatively straightforward to design a many-rotor eVTOL that carry a small payload (less than 500 pounds) over short distances. For larger payload weights and commercially desirable ranges the strategy of using many rotors becomes increasingly problematic. Using a larger number of smaller rotors provides less disc area than fewer big rotors, requires more power per aircraft weight, is difficult to hover at low noise because the lower total rotor disc area results in higher blade tip Mach numbers, or in larger number of wide-chord blades, or both, and makes autorotation flight after loss of power more dangerous, because the autorotation descent rate increases in proportion to the square root of rotor disc loading and recovery from high descent rate is risky.
Using a small number of bigger rotors (two, three or four) could solve some of the problems discussed above, but that approach is completely contrary to the prevailing wisdom. Among other things, the characteristics needed to optimize vertical lift are very different from the characteristics needed to optimize forward flight. In addition, those of ordinary skill in the art would dismiss the idea of having fewer rotors on the grounds that doing so would unacceptably sacrifice safety in the event of motor failure of any of the rotors, and would introduce unacceptable inefficiencies for an eVTOL. Still further, these problems cannot adequately be resolved by having separate lift and forward propulsion systems.
What is still needed is a vertical takeoff and landing rotorcraft that can safely carry a payload of at least 500 pounds, using a unitary lift and forward propulsion system with no more than four rotors, all while powered by the current technology battery.
All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention. Unless a contrary meaning is explicitly stated, all ranges are inclusive of their endpoints, and open-ended ranges are to be interpreted as bounded on the open end by commercially feasible embodiments.
Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.
As used herein, and unless the context dictates otherwise, the term “coupled to” is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms “coupled to” and “coupled with” are used synonymously.
The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.